1. Field of the Invention
The present invention relates to a high voltage shielding design as defined by the preamble of claim 1, and particularly originates from a desire and requirement to apply known high voltage technology in a modern charged particle lithography system.
Such shielding designs are generally encountered in the art of high voltage engineering. In known constructions this is often realized by placing positive and negative poles of an electrical circuit at a certain and fairly large distance apart to prevent electrical breakdown. In other solutions the electric discharge path is elongated, e.g. by providing undulations and other kind of irregularities in said path.
Both known types of solution in practice lead to either or both of a voluminous and an elaborate construction. This is in particular in the technologically highly intensive, and therefore cost and capital intensive environment of lithography highly undesired. It is therefore an object of the present invention to realize a considerably less space consuming and/or a relatively simple shielding design, in particular for enabling high voltage parts in a lithographic environment, more in particular within a charged particle lithography machine.
2. Prior Art
Such a shielding design is particularly suited for, but not limited to, use in charged particle beam projection systems for maskless lithography. Such systems are generally known and have the advantage of fabrication on demand and possibly lower tool cost, due to a lack in necessity to use, change and install masks. One example of such a system, disclosed in WO2007/013802, comprises a charged particle column operating in a vacuum chamber with a charged particle source including a charged particle extraction means, a means for creating a plurality of parallel beamlets from said extracted charged particles and a plurality of electrostatic lens structures comprising electrodes. The electrostatic lenses serve the purpose of focusing and blanking the beamlets, where blanking is realized by deflecting one or a multiplicity of such charged particle beams to prevent the particle beam or multiplicity of beamlets from reaching the target such as a wafer. For realizing the final part of the projection on said target of a computer based image pattern non blanked beamlets are, at a final set of such electrostatic lenses projected onto said target.
Said charged particle column also requires a multiplicity of electrical leads to be fed to the column as to provide signal access. For providing said signal access to the vacuum chamber it is generally necessary to provide feed-throughs which pass said electrical leads through the vacuum chamber wall for providing a electric coupling between the vacuum chamber and the outside environment. Said electrical leads may be required to supply and sustain high voltage signals.
Where high voltage, usually an electric potential of more than 1 kV, is used, it is generally necessary to provide sufficient electrical insulation and shielding to prevent the high voltage signals from either electric breakdown or causing electron creep to occur.
Electrical breakdown occurs when the electric potential between the positive and negative poles is sufficiently high that the electric field generated causes a discharge from one pole to another through a space separating the poles.
Electron creep occurs when individual electrons migrate across a surface in between the positive and negative poles, said electrons being effectively extracted from the negative pole. This effect becomes more notable as the electric potential increases to higher values or when the metal parts are very thin, such as is the case when conductive coatings are used.
Both these phenomena are more likely to occur when field enhancement occurs, as an effect of the geometric configuration of the electric field. Where the electric field and corresponding electric field lines, or equipotential lines, are normally evenly spaced resulting in a constant and uniform field strength, distortions in the field geometry caused for instance by protrusions or sharp edges effectively push the equipotential lines together, locally increasing the strength of the electric field. This increased strength of the electric field will increase the likelihood of electrical breakdown and electron creep. In a typical application the electric field may reach 10 kV/mm, whilst in certain high-performance charged particle applications systems field strengths of up to 30 kV/mm occur.
To prevent the occurrence of the previously mentioned phenomena commonly a large enough distance is maintained between both poles to prevent breakdown, meaning that such a shielding design inherently is of a large size. This size disadvantage may be evident in spacing the poles apart in a planar fashion and resulting in a large diameter, meaning that for instance ports in a vacuum chamber need to be larger than desired. Alternatively, such a shielding design can space the poles perpendicular to the plane, resulting in an increase in volume occupied by the shielding.
One such practically embodied shielding design is known from U.S. Pat. No. 4,231,003 wherein a coaxial vacuum feedthrough is disclosed. The known feedthrough comprises a first round metal bar with one vacuum end and one atmosphere end for connecting an external and an internal wire. The metal pin is enclosed by a first ceramic cylinder which in turn is enclosed in an airtight manner by a first metal cylinder. A second ceramic cylinder encloses the first metal cylinder. A second metal cylinder airtightly encloses the second ceramic cylinder and is airtightly fixed to the vacuum vessel. In this manner a feed through is provided for one wire whilst maintaining a vacuum seal and providing electrical insulation between the vacuum vessel and the electrical signal.
In another field of high voltage isolation, U.S. Pat. No. 7,045,794 describes a stacked lens structure and a method of use thereof for preventing electrical breakdown. In the stacked lens structure, which comprises conductive layer and insulating layers between the conductive layers, recesses are made to increase the length of the breakdown path at surfaces where electrical breakdown is likely to occur. Furthermore, serrations may be formed in the recesses to further increase the surface breakdown path length. In another embodiment, silicon lenses are formed with cutouts. This solution relies on increasing the length of the surface breakdown path.
Other shielding designs are known from for example U.S. Pat. No. 5,117,117 and U.S. Pat. No. 4,176,901.